Chinook salmon (Okanagan population) COSEWIC assessment and status report: chapter 7

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Habitat

• Habitat Requirements
• Habitat Trends
• Habitat Protection and Ownership

Habitat Requirements

Chinook salmon rearing occurs in streams, as well as lakes, estuaries, and the ocean. Maturing adults migrate to spawn in their natal streams, whereupon the adults die. However, precociously maturing fish, especially sub-yearling males, may be able to recover from spawning and spawn again in a subsequent year (Mullan et al., 1992).

While chinook spawning habitat includes a broad range of water depths, water velocities, and substrates (e.g., Scott and Crossman, 1973; Healey, 1991), it is often patchily distributed within apparently uniform habitats, suggesting that other factors, such as intra-gravel flow, may be most critical (M. Healey, personal communication, 2004). In some cases, however, water velocity and substrate have been found to be useful predictors of preferred chinook spawning habitat (Gallagher and Gard, 1999). Spawning habitat characteristics for Okanagan chinook, based on measurements taken at 18 redds in 2002 and 2003, have been documented (Table 1; Phillips and Wright 2005).

Phillips and Wright (2005) estimated that there could be spawning habitat for between 1260 and 4340 spawning pairs of Okanagan chinook between Osoyoos Lake and McIntyre Dam. The upper end of the habitat availability range is based on the above factors and is a likely over-estimate; however, the lower end of the range also considers areas of groundwater inflow in the river and observed redd associations with instream bars and islands.

Estimates of the area of the Okanagan River and Osoyoos Lake that are available for spawning and juvenile rearing suggest that the geographic extent of occurrence (EO) of Okanagan chinook is roughly 16 km², and that the area of occupancy (AO) is only slightly less at perhaps 15 km² in Canadian freshwaters. Anadromous individuals also use the Columbia River as a migration corridor, and the Pacific Ocean for growth to adulthood (neither of which are included in calculations of EO and AO).

Habitat Trends

Stream Habitat

As in the United States, chinook salmon in Canada have been adversely affected by numerous factors, including water withdrawals, construction of dams (for power generation or water diversion) that limit fish passage or entrain/harm migrating fish, and degradation of habitat through industrial, agricultural and urban usage (Raymond, 1988; Myers et al., 1998). Habitat loss/degradation might be inferred, in part, from the historical decline in chinook salmon abundance in the Upper Columbia River (i.e., hundreds of thousands of chinook salmon in the Upper Columbia River in the mid-1800s vs. about 58,000 in the same area in recent years; Myers et al., 1998).

Loss of habitat (or access to habitat) for spawning, rearing and holding has had an unknown impact on the Okanagan chinook population. Had access been restricted while the run was still strong, the impacts would likely have been profound. However, the run was probably already depressed, possibly by factors outside the basin, by the time the mainstem dams in Canada were constructed. Historical reports and Okanagan traditional knowledge indicate that chinook had originally had access to Okanagan Lake (Clemens et al. 1939, Ernst 1999, Vedan 2002).

The long history of major mainstem channel modifications in the Canadian Okanagan basin began in about 1910, with modifications to the outlet of Okanagan Lake (Symonds, 2000). Since that time, dams have been constructed at the outlets to Okanagan Lake (Penticton Dam), Skaha Lake (Okanagan Falls Dam), Vaseaux Lake (McIntyre Dam), and Osoyoos Lake (Zosel Dam in the U.S.). Only Zosel Dam at the outlet to Osoyoos Lake is regularly passable to upstream migrating fish. While McIntyre Dam can be managed to permit passage of adult salmonids at some times of year (Summit and ONFC, 2002), this is not the usual practice. At present, all anadromous salmonids spawn downstream of McIntyre Dam, unless a few are permitted to pass the dam.

In addition to loss of access to habitat, there have been major direct losses of spawning and rearing habitat in the Canadian Okanagan River. Most of the river between Okanagan and Osoyoos Lakes has been straightened and channeled (Symonds, 2000). Where there once was over 10 km of channel (about 80 000 m²) between Okanagan and Skaha Lakes that was suitable for use by spawning sockeye (and presumably chinook), there is now only about 3 km of channel suitable for spawning (Anonymous, 1909; Summit, 2003). Similar or greater habitat losses have occurred throughout the mainstem Okanagan River in Canada wherever the river has been channeled (Hourston 1954): one estimate puts the loss of natural accessible river channel at 91% (Bull 1999). However, this does not necessarily mean that there has been an equivalent reduction in suitable habitat for spawning salmonids. The amount of summer rearing habitat in the river (i.e., groundwater-fed side channels) that has been lost is unknown. However, it is likely that little usable summer habitat remains in the dyked sections of channel due to the absence of side channels and other areas where groundwater inflow may have a significant temperature-moderating effect.

At present, the Okanagan River is used by spawning Okanagan chinook and may be used by rearing juveniles for a period ranging from days to months or longer. High water temperatures in the river limit the period when mature Okanagan chinook can enter the river, both for migration and spawning, and limit the area available for rearing juveniles. During the summer months, water temperatures in the river are often in the lethal range for chinook salmon, except in groundwater-fed side channels (ONA, 2003). Juvenile salmonids have been observed in side channels of the river when temperatures in the rest of the river were 24 °C (Alexis et al. 2003). Water temperatures in the river prior to construction of the mainstem dams and other channel modifications are not known.

Following the last major channel modifications in the 1950s, fish habitat in the river has remained relatively unchanged until the past five years. Stream channel habitat conditions do not appear to have degenerated in the past 50 years. In fact, water quality in the river has probably improved in the past 20 years in response to widespread improvements to sewage treatment in upstream areas. Other habitat improvements include the addition of fish screens to many water intakes on the river and the experimental addition of rock riffles (to increase habitat diversity and improve fish passage) in a channeled section of river. Another major improvement is a water management initiative directed at improving decision-making for the benefit of fish in the mainstem river and lakes (COBTWG, 2004). The water management initiative is expected to significantly improve sockeye--and presumably chinook--smolt production (COBTWG, 2004). Lastly, there are conceptual plans to re-naturalize the Okanagan River to its fullest extent, with measures including re-oxbowing, set-back dyking, riparian restoration, and construction of instream riffles (Gaboury et al. 2000). Implementation of this concept has begun with an area along the Okanagan River purchased and identified for set-back dyke restoration.

Lake Habitat

Due to historical dam construction and current dam management practices Osoyoos Lake provides the only lake-rearing habitat available to anadromous salmon in the Okanagan basin. Adult Okanagan chinook hold in the lake for weeks or months prior to spawning, and juveniles rear in the lake for a period ranging from weeks to years. Most of the detailed observations on rearing conditions in Osoyoos Lake have been focused on sockeye salmon, but much of this information also applies to Okanagan chinook.

Osoyoos Lake consists of a series of three basins, with the southern-most basin spanning the Canada-United States international boundary. The lake has a mean depth of 15 m and a water residency time of about 0.7 years (Pinsent et al. 1974). It is classified as a mesotrophic lake, with phosphorus as a limiting nutrient (Wright 2002). There has been no paleolimnological work conducted on Osoyoos Lake, so historical information is lacking. However, phosphorus reduction measures were implemented (as in the other mainstem Okanagan lakes) in the early 1970s due to water quality issues (Pinsent et al. 1974). Jensen and Epp (2001) note that water quality has improved in terms of phosphorus reduction: where spring phosphorus loading in the early 1970s was about 25-30 µg/L, today it averages 15-20 µg/L.

Osoyoos Lake has a high percentage of littoral area (23%) compared to Skaha and Okanagan Lakes (15%) (Wright, 2002). However, high epilimnetic water temperatures likely limit littoral habitat usage by chinook during the growing season (April to November). In addition, exotic Eurasian milfoil (Myriophyllum spicatum) has spread rapidly in Okanagan littoral areas and would provide additional habitat for exotic “ambush” predator species such as largemouth bass (Wright et al. 2002). Beach seining for sockeye in the north basin of Osoyoos Lake in 2002 found that fry were present in low abundance in littoral areas until water temperatures exceeded 17 °C (mid-June), at which time they move offshore (ONAFD, unpublished data, 2005). In addition, rearing habitat in the lake may be further constrained in late summer and fall by high water temperatures in the epilimnion and low dissolved oxygen levels in the hypolimnion (Wright 2002; Wright and Lawrence 2003). This habitat squeeze between high water temperatures in the epilimnion and low dissolved oxygen levels in the hypolimnion is particularly severe in the two southern basins of the lake, such that all sockeye (and likely Okanagan chinook) production in Osoyoos Lake is from the northern basin (Hyatt & Rankin, 1999). However, in spite of habitat limitations, sockeye smolts in Osoyoos Lake are among the largest in the world (Hyatt & Rankin 1999), testament to the abundance of food in the lake. Okanagan chinook rearing in the lake may exhibit similarly high growth rates, which may contribute to the anomalous extended freshwater rearing (Brannon et al. 2004).

An additional constraint of unknown severity results from the introduction of an exotic shrimp, Mysis relicta, into the Okanagan basin. M. relicta has been present in the lake since at least 1998 (Hyatt and Rankin 1999), having invaded from upstream lakes where it is well established. There is usually a downward trend in limnetic fish populations after M. relicta invasion (Lasenby et al. 1986), and this has already been documented for kokanee salmon populations in Okanagan Lake. While chinook are more often found in littoral areas and feed less on zooplankton (i.e., less competition with mysids), this behaviour has not been established for Osoyoos Lake, where the littoral zone is likely inaccessible through much of the growing season due to high water temperatures (ONAFD, unpublished data 2005).

In summary, high water temperatures in the Okanagan River constrain the timing of Okanagan chinook adult migration and spawning, and restrict juvenile rearing habitat to groundwater-fed side channels during much of the summer. Construction of dams has reduced accessible Okanagan chinook habitat to a fraction of its former size, eliminating access to the mainstem river, lakes and tributaries upstream of McIntyre dam (i.e., the outlet of Vaseaux Lake). In addition, approximately 90% of the mainstem Okanagan River in Canada has been modified, resulting in a major loss of spawning and rearing habitat. Salmon rearing habitat in Osoyoos Lake is largely limited to the north basin. Okanagan chinook may rear in littoral areas early in the growing season, but are soon constrained by high epilimnetic water temperatures. Rearing habitat is further constrained by low hypolimnetic dissolved oxygen levels in the late summer and fall. In addition, Okanagan chinook rearing in the lake may have to compete with mysids for food (at least until they become piscivorous), especially when littoral areas are inaccessible, and may be subject to predation by exotic species when rearing in littoral areas. However, in spite of these constraints, Osoyoos Lake does provide a productive environment for salmon rearing, as evidenced by the growth rate and smolt sizes of sockeye salmon.

Habitat Protection and Ownership

The main chinook spawning area in the Okanagan River is between Oliver and McIntyre Dam. Nearly the entire accessible spawning area in Canada is dyked, and thus the channel is either actively managed by the B.C. Ministry of Water, Land and Air Protection or is within the boundaries of Osoyoos Indian Band reserve lands. There is very little development along the river channel where it passes through the Indian Reserve.

In the Canadian portion of the Okanagan and Similkameen watersheds, provincial parks account for 14.5% of land ownership. Indian Reserve lands comprise an additional 4%, and 50.2% is municipal or privately owned land. The remaining land base (approximately 30%) is designated as Crown lands. In addition, 31.8% of the land base (distributed throughout these designations) is held in the Agricultural Land Reserve.